The present invention relates generally to the detection of imbalances in a paired line. More particularly the present invention relates to the accurate measurement of pair balance in the presence of a high level of power influence or other interference. More particularly it relates to the detection of small pair imbalances that can affect the higher frequencies communicated by ADSL and VDSL circuits but may not be detectable with voiceband tests.
Paired lines are a conventional means of carrying telecommunications transmissions. A paired line is made up of two balanced conductors individually insulated and twisted together. Paired lines are typically bunched together in a cable termed a paired cable which contains up to one hundred or more paired lines, wherein each paired line is capable of independently carrying telecommunications signals. Paired lines are generally effective telecommunications carriers. However, it is not unusual for noise to occur in paired lines which is extremely disruptive to the clarity of the transmitted signal.
When noise is reported in a paired telecommunications line, correction of the condition requires confirming the presence of the noise in the line by measuring its level and then isolating and locating the noise source for purposes of eliminating it. There are a wide range of noise sources for which detection is desirable since virtually any condition which can cause an imbalance between two conductors of a paired line can result in noise. Among the causes are series resistance faults, shunt resistance faults, cross faults, shunt capacitance faults, unbalanced series inductance, and power influence. Series resistance faults occur when there is an open in a line, often resulting from a corroded joint. Shunt resistive faults occur when another body grounds a paired line. Cross faults occur when there is communication between adjacent paired lines in a cable. Shunt capacitance faults occur when one conductor of a pair is slightly longer than the other conductor, and the longer conductor possesses a higher capacitance to ground than the shorter conductor. Unbalanced series inductance occurs when only one half of a load coil is connected to a paired line at some point along the length of the line. Power influence is induced voltage from an ac power source adjacent the paired line. Unlike the above-recited causes of imbalance, power influence imbalance can occur even when the paired line is free of faults and appears balanced in the absence of the power influence.
Power influence, which as noted above is induced voltage from line to ground, most commonly occurs when the paired line is near a power line. In the United States, the power line frequency is typically 60 Hz, but power influence can likewise result from other power line frequencies, including 50 Hz, as typically found in many other parts of the world. Power influence can create unique problems for noise detection when it occurs in conjunction with a fault. For example, a series resistance fault may only produce a high level of noise when accompanied by a high power influence. Therefore, a noise caused by the fault may be observed by a user at a time of high power demand on a nearby power line, but when a repairman is dispatched to the site, the power demand and correspondingly the power influence may have diminished so that the noise resulting from the fault alone is no longer detectable by conventional detection devices. Accordingly, such a fault is very difficult to locate and repair.
Another detection problem results from the fact that power influence signals often do not create large longitudinal current flow. Such flow is necessary to detect series resistance faults because longitudinal current flow through a series resistance fault produces a voltage imbalance in the paired line which can be measured metallically. However, because conventional passive detection devices lack the ability to independently generate longitudinal current flow, they accordingly may fail to detect such faults where power influence is relied upon to generate longitudinal current flow.
Various attempts have been made to detect imbalances in paired lines. For example, the “Stress Test” has become the accepted name for the test described in U.S. Pat. Nos. 5,157,336 and 5,302,905, which provided a new way to test all cable pairs, working or dry, for proper balance. This test has become the telephone industry standard for determining the usability of a pair before placing it in service, and for isolating pair balance trouble to the source. A particular benefit of the Stress Test is in testing dry pairs before placing them into service as the test identifies “killer pairs” that tested good by previous methods yet tend to go bad within 48 hours after being placed in service.
Apparatus implementing the above Stress Test sent out a simplex (both sides of the pair excited equally with respect to ground) “Stress” tone through a balanced center tapped termination. Any unbalance on the pairs converted the simplex tone to metallic (across the conductors) which was amplified and filtered through a C Message filter. The filter output was converted to display either stressed noise or stressed balance, with stressed noise in dBrnC being the most popular.
The Stress Test simplex stress tone acted as an artificial “Power Influence” signal, permitting any pair's balance to be tested, even those pairs having too little power influence to allow a normal Longitudinal Balance reading. Longitudinal Balance readings expressed the difference between passive Power influence and Noise Metallic readings on the pair and thus did not place simplex excitation on the pair. The Stress Test internal termination to ground caused longitudinal current flow on the pair, revealing series resistance unbalances invisible to the Longitudinal Balance test. The pair can be tested from either end and does not require a termination in the C.O.
A problem exists with the above Stress Test in that Induction noise induced onto the tested pair in the voice band adds to the test signal converted from simplex to metallic by any unbalance on the pair causing high stressed noise inferring poor pair balance when balance is not the source. In addition, high Power Influence can swamp out the applied simplex stress voltage causing erroneous high stressed noise readings. In areas with high power influence approaching or above the applied “Stress” voltage, the Stress Test will erroneously read bad on good pairs. Thus, on noisy pairs you may not be measuring stressed noise, but induced noise converted from high Power Influence (50/60 Hz harmonics) on the pair due to the wideness of the C Message filter. This erroneous reading can cause technicians to try to improve pair balance rather than correcting high Power Influence, the true cause of the bad Stress Test indication. Therefore a “Voiceband Stress Test” is needed that can indicate the true stress balance of a pair in the voiceband with the presence of normal or high power influence.
Furthermore, the Stress Test as described above applies a simplex tone, in the voiceband typically near 1 kHz and indicates the balance of the pair at that frequency. Pairs that stress bad in the voiceband usually will not perform in the DSL band. A good Stress Test reading however, does not necessarily indicate the pair will perform well in the DSL band. Minor capacitive or resistive unbalances that do not give a bad reading in the voiceband, can be service-effecting in the DSL bands.
Therefore, a test is needed that performs like the Stress Test in the DSL band, so it can be used to isolate service effecting DSL problems by technicians already familiar with using the Stress Test. Preferably, this test would quickly give a numeric readout allowing a confirmation that the pair is within parameters for service. It would be desirable if the test were faster than a TDR reading, not requiring the technician to interpret whether a TDR squiggle seen at the far end of a pair after he turns up the gain will affect service. Furthermore, the improved test apparatus would work on an in-service DSL circuit where a TDR will not work.